This invention relates to an apparatus for controlling an internal combustion engine operable on gasoline fuel or gasoline/alcohol fuel blend.
For example, Japanese Patent Kokai No. 56-98540 discloses an apparatus for controlling an internal combustion engine operable on gasoline fuel or gasoline/alcohol fuel blend. The engine control apparatus employs an alcohol concentration sensor sensitive to the alcohol concentration of the fuel delivered through a fuel injector to the engine. The sensed alcohol concentration is used in calculating an appropriate value for fuel delivery requirement in the form of fuel-injection pulse-width. The engine control apparatus also employs an oxygen sensor sensitive to a deviation of the air/fuel ratio from a stoichiometric value. The sensed deviation is used in calculating a correction factor for correcting the fuel delivery requirement value to provide a closed loop air/fuel ratio control. The correction factor is set within a control range defined by predetermined upper and lower limits, for example +25% with respect to a value obtained at a stoichiometric air/fuel ratio. If a great difference exists between the alcohol concentration of the fuel sensed by the alcohol concentration sensor and the alcohol concentration of the fuel residual in the fuel injector just after the engine is refueled, the air/fuel ratio will deviate greatly from stoichiometry, causing the correction factor to be clamped to the upper or lower limit. With such a conventional apparatus, however, this clamped state cannot be released in a short time sufficient to avoid an unstable engine operation which would eventually caused the engine to stall.
In addition, the conventional apparatus is arranged to maintain the correction factor at a fixed value so as to suspend the closed loop air/fuel ratio control when the engine is idling. Although this arrangement is effective to avoid an unstable engine operation at idle conditions, its effectiveness is limited or invalidated when the sensed alcohol concentration changes to a great extent.